The present invention relates to combustion products detectors and, in particular, to means for testing the sensitivity of such detectors.
Devices for detecting combustion products, such as smoke, are principally of two types, viz., ionization-type detectors and photoelectric detectors. The principles of the present invention are applicable to any type of combustion products detector, although the preferred embodiment is described in connection with an ionization-type detector
In an ionization-type detector, the sensor is typically an active ionization chamber which is relatively open to ambient air. A reference impedance is typically provided by a reference ionization chamber which is relatively closed to ambient air, a reference chamber which is open to ambient air but insensitive to products of combustion, or a physical resistor. Each of the chambers includes a pair of spaced electrodes, or the chambers may share a common electrode therebetween, and means are provided, such as a source of radioactive energy, for ionizing air molecules between the electrodes. With the chambers or chamber and resistor in series to form a voltage divider, and with a voltage applied thereacross, an electric field is generated between the electrodes to establish a current flow through the chambers or chamber and resistor by movement of ions between the electrodes. The potential at a sensing electrode at the junction between the active chamber and reference impedance is then in accordance with the relative impedances of the two elements.
A change in the ambient conditions, such as the presence of combustion products, affects the ion current flow through the series elements and therefore the impedances thereof. The voltage at the sensing electrode is monitored by a detection circuit and when it exceeds a preselected alarm level, the detection circuit energizes a suitable alarm circuit. In self-contained, i.e., battery-powered, combustion products detectors of this type, it is known to provide a battery monitoring circuit which will cause a low battery signal to be generated when the battery has been depleted to near a level at which successful operation of the alarm circuit is no longer assured.
It is known to provide in combustion products detectors means for testing the operation thereof. In particular, means have been provided for testing the sensitivity of the combustion products sensor by simulating the presence of combustion products. In ionization-type detectors, this test means may comprise a manually-operated switch for connecting an impedance across the ionization chambers, thereby to change the voltage thereacross so that the sensing electrode voltage is equal to that which would be produced if combustion products were present in an amount beyond which the generation of an alarm is necessary. Such arrangements are disclosed, for example, in U.S. Pat. Nos. 4,097,850 and 4,246,572. These test devices simply check to see if the sensitivity of the sensor is above a predetermined minimum sensitivity. But it is important that the sensitivity not be too high, so as to avoid frequent false alarms. No prior test devices are concerned with the maximum sensitivity of the sensor.
All such manually operated test devices rely upon the user to remember to test the combustion products detector at regular intervals. But users frequently forget to make such tests. Furthermore, since combustion products detectors are typically located on ceilings or other relatively difficult to reach locations, manual testing of the device may be sufficiently inconvenient to deter the user from making such tests.
It is also known to provide a circuit for selfchecking of the sensitivity of a combustion products detector. Such an arrangement is disclosed in U.S. Pat. No. 4,306,230 and No. 4,302,753, which disclose continuous monitoring of the clear-air voltage from the sensor. But those devices necessitate an additional voltage comparator for the clear-air voltage monitoring, and they check only to be sure that the sensitivity is above a minimum sensitivity level. Furthermore, the device of Pat. No. 4,302,753 may not properly check this minimum sensitivity level since the sensing electrode voltage changes with various smoke levels and is typically nonlinear due to chamber saturation, and large errors in tested smoke levels may therefore result.
There have also been provided various types of fault detection circuits which automatically periodically conduct a self-test of a system in order to detect the presence of certain faults. Such systems are disclosed, for example, in U.S. Pat. No. 3,928,849 and 4,199,755. But none of these periodic self-testing circuits are designed for testing sensitivity of a combustion products detector. Typically, prior combustion products detectors which test sensitivity by simulating combustion products, operate on the principle that the normal smoke alarm should operate under the test conditions. If the alarm fails to operate, due to improper sensitivity of the sensor, a threat to life safety will occur. There is, therefore, a need for a combustion products detector which will provide an automatic, positive indication when the sensitivity fails to meet the test criteria for proper operation.